The present invention relates to a method and a device for creating a correction value table, for determining a test variable, and for identifying the pressure loss in a tire of a wheel.
A method for identifying pressure loss of this type is disclosed in DE 19 721 480 A1.
Basic physical correlations are explained by way of FIG. 6. Reference numeral 61 represents a regular wheel on the roadway 60. The wheel center 63 moves with the vehicle chassis and, thus, at the vehicle speed Vf. Corresponding to the generally accepted relationship between the track speed v of a point on a disc rotating with the angular speed xcfx89, with the said point being spaced from the center of rotation by the radius R, i.e., xcfx89=v/R, xcfx89r=vF/Rr results on the left side of FIG. 6. The angular velocity xcfx89 of vehicle wheels can be determined by means of wheel sensors, while the vehicle speed v generally cannot be sensed by sensors. The dynamic rolling circumference of a wheel varies in the event of pressure loss. The wheel rotates faster compared to the normal condition or compared to the wheel without pressure loss.
Additional effects can influence the angular velocities of the wheels, but the resulting difference between the angular velocities of individual wheels would not indicate pressure loss in any one of the wheels. Examples herefor are traction slip, different geometries during cornering, unsymmetrical load distribution in the vehicle, or similar factors. Two effects which result drom driving dynamics, especially during cornering or in the traction case, will be explained referring to FIGS. 1A to 1C.
FIG. 1A shows a vehicle 10 with left front wheel 11, right front wheel 12, right rear wheel 13, and left rear wheel 14. The vehicle rides at a speed vF in a curve to the right, and it is assumed that the vehicle""s point of gravity S follows the radius R about the center M. The wheels 12 and 13 on the right-hand side of the vehicle roll on the inner track and, therefore, have a track with approximately the same smaller inside radius Ri, while the wheels 11 and 14 on the left-hand side of the vehicle roll on the outer track and, hence, ride in a curve with the larger outside radius Ra. Because they have to cover a larger distance in the same time, the curve-outward wheels 11 and 14 exhibit a higher track speed and, thus, also a higher angular velocity than the curve-inward wheels 12 and 13. However, these differences are not due to a pressure loss in any one of the wheels.
Another effect is explained with reference to FIG. 1B. The vehicle shown in FIG. 1A is shown from the back, it follows the same track as the vehicle shown in FIG. 1A (curve to the right, i.e., to the right into the drawing plane in FIG. 1B) about the center M with radius R. A centrifugal force Fz which is applied to the vehicle point of gravity S is produced due to the cornering maneuver. The counterforce is the friction force Fr between the vehicle wheels and the roadway. Because these forces do not act in the same plane, a rolling moment Mr is produced in the mentioned situation counterclockwise about the longitudinal axis of the vehicle. This brings about that the curve-outward wheels 11 and 14 are subjected to greater stress than the curve-inward wheels 12 and 13. The result is that they are more compressed, hence show a smaller dynamic rolling radius and a higher angular velocity. The effect of FIG. 1B points to the same direction as the one described by way of FIG. 1A so that they add.
FIG. 1C shows a situation in which the vehicle 10 moves on the roadway 15 driven by engine 16. In the example of FIG. 1 the rear axle is driven so that the wheels 13, 14 of this axle will exhibit both traction slip and brake slip, while the wheels of the front axle 11, 12 can only exhibit brake slip. Especially in the case of traction, the wheels 11, 12 of the front axle roll freely and, hence, have an angular velocity xcfx89=vF/R, while the wheels of the rear axle frequently have a higher amount because the wheel slip xcfx89s adds to the above-mentioned amount xcfx89. Likewise this effect has nothing to do with different angular velocities due to pressure loss in any one of the tires.
In view of the above, it is important to eliminate disturbing effects according to FIGS. 1A to 1C. In this respect, DE 19 721 480 A1 discloses a method wherein wheel speeds are added in pairs, the sums are brought into a relation to each other, and the value of the quotient is checked. More particularly, a method is disclosed wherein the wheel speeds of the wheels lying on a diagonal are added and the results achieved are divided. A quotient will thus be calculated which differs more or less from the ideal value 1 (constant velocity of all wheels). When especially a tire with pressure loss exists, a considerably lower value will occur either in the numerator or the denominator of the fraction so that, for this reason, the resulting quotient will also differ greatly from the ideal value 1, upwards or downwards. Further tests may then be performed in order to detect a wheel with pressure loss, if there is one. Effects of curve geometries or traction slip are frequently compensated by considering or summing the values of diagonally opposite wheels. On the other hand, this compensation not always occurs with certainty so that comparatively wide tolerances must be chosen to avoid wrong detections. The result is that the detection occurs only at a relatively late point. During cornering maneuvers, for example, differential locks may prevent the compensation of effects due to different curve geometries. When one axle is locked, the wheels of the axle roll with the same track speed and angular velocity so that they cannot contribute to compensating the unbalance in the summation to the respective other partner.
An object of the present invention is to provide a method and a device for creating a correction value table, for determining a test variable, and for identifying the pressure loss in a tire of a wheel, permitting a reliable detection of pressure drop in a tire.
This object is achieved with a method for creating a correction value table, for determining a test variable, and for identifying the pressure loss in a tire of a vehicle, wherein the test variable is a quotient of each two sums of two wheel radii or variables mirroring these wheel radii, comprising the steps
determining a driving dynamics variable of the vehicle, and
determining a correction value for the test variable and storing the said in dependence on the value of the driving dynamics variable which prevailed during the correction value determination.
In the method for creating a correction value table for a test variable for identifying a pressure loss in the tire of a vehicle, individual correction values are determined for the test value and stored in dependence on the value of a driving dynamics magnitude which prevailed during or at the point of time of the correction value determination. A table of correction values is this way prepared in the course of time. The input variable of the table is the driving dynamics variable, the output value is thus a driving-dynamics responsive correction value so that the test variable for identifying a pressure loss in the tire of a vehicle can be corrected in dependence on driving dynamics.
The determination of the correction value is a learning operation. The correction value can be determined when the vehicle dynamics, in particular the driving dynamics variable satisfies defined conditions with respect to their values or with respect to their time variations, no matter whether absolute or relative. More particularly, the demand may be that the vehicle dynamics or especially the driving dynamics variable referred to has a certain constancy (within a range of values within a time window), or that the variation of driving dynamics is lower than a threshold value. The test variable may be determined from several wheel radii or quantities which correspond to these wheel radii. For example, the test variable may be a quotient of two sums of two wheel radii each.
The possible range of values of the driving dynamics variable can be subdivided into ranges. When the value of the driving dynamics variable is represented digitally, the range division may occur already due to the digital quantization. Correction values may be determined as described hereinabove for individual or several values of the driving dynamics variable. For other values of the driving dynamics variable, correction values may be determined by interpolation with appropriate methods (linear, in general polynomial).
The driving dynamics variable may be a wheel torque and/or a curve characteristic value. The test variable may be determined from the variables of several wheels of the vehicle. In particular, it may be the quotient of two sums of such variables.